Replying to Need help buliding a box, any welders?

what i cant understand is how to make the pipe flush with the poly. im thinking of making an 2 by 10ft outer rail with 2in pipe lay the pipe on the floor and place the poly down in the center then cut 6 or so pieces of angle iron in 2ft strips, lay the angle iron on top of the ploy and tack the iron to the pipe, just like a ladder so when you flip everything over the poly's flush with the pipe and ready to screw down. is this how they are in the park ? and do the have plywood under the poly to screw the ploy to the plywood or just to the angle iron?